Generally, the actuator is a device which is coupled to a desired unit and provides a driving unit with a forward or backward force and/or a rotating force generated by the hydraulic or pneumatic source. The actuator according to the present invention may be applied to both hydraulic and pneumatic actuators, but for convenience, all description and explanation will be given for the pneumatic actuator, hereinafter.
FIG. 1 depicts an example of actuator according to the prior art, in which a pneumatic actuator for operating a swivel valve is shown. As seen from the drawing, the actuator comprises a pair of piston 160 positioned at both side of interior of cylinder 100; a rotating axis 140 rotating in either direction by the forward or backward action of piston 160; the first air pathway 103 connecting the space 101 between the pistons 160 to the exterior of cylinder; and the second air pathway 104 connecting the space 102 between the pistons 160 and the corresponding cylinder end wall to the exterior of cylinder, thereby opening and shutting the valve connected to the rotating axis 140 by each forward or backward action of the opposed pistons 160 by means of the direction of the compressed air provided through the first and second air pathways 103, 104.
In this actuator, if any external device is coupled with the rotating axis 140, an additional control unit is necessarily required to adjust an action range of the external device such as the angle of valve opening. This control unit comprises of a sensor to detect the displacement of the rotating axis 40, and a controller to control the action of the solenoid valve 20 according to the input data from the sensor and input means to set the action range of the device. And the cost of all these components is expensive, which results in high cost in installing the actuator.